1) Field of the Invention
The present invention relates to an exhaust gas cleaning device constructed of a metal-made honeycomb core body for carrying an exhaust gas cleaning catalyst. As a cleaning means for exhaust gas from a motor vehicle, the exhaust gas cleaning device is generally installed at an intermediate point of an exhaust pipe.
More specifically, this invention is concerned with an exhaust gas cleaning device of the above sort, which is employed under severe conditions. The exhaust gas cleaning device features the use of a metal-made honeycomb core body with improved resistance to deformations and breakage by thermal expansion and stress.
2) Description of the Related Art
Exhaust gas cleaning devices of the above sort have conventionally been fabricated in the following manner. Namely, a planar band made of a heat-resistant steel sheet and a corrugated band obtained by forming a similar steel sheet into a wavy or sinuous form are superposed one over the other in a contiguous relation and rolled together into a multi-layered spiral form or are superposed in, layers, thereby forming a honeycomb-shaped multi-layered composite body (hereinafter called the "honeycomb core body") axially defining a number of network-patterned gas flow passages for allowing exhaust gas to flow therethrough. The honeycomb core body is then inserted into a cylindrical metal casing which are opening in both ends thereof.
The honeycomb core body and metal casing are thereafter firmly joined together by brazing or welding, so that they can withstand thermal expansion and stress due to the high temperature of exhaust gas, exothermic reactions of exhaust gas induced by the catalyst and the like and further vibrations while the associated motor vehicle is running. Needless to say, the planar and corrugated bands which make up the honeycomb core body can be firmly joined together at contacts therebetween by any one of various suitable methods.
There is however a recent move toward constructing an exhaust gas cleaning device without a metal casing which is adapted to receive and firmly fix a metal-made honeycomb core body, in other words, with the metal-made honeycomb core body alone from the stand point of the price competition with conventional cordierite ceramic carriers. The omission of such a metal casing naturally leads to a reduction in the cost for inserting a metal-made honeycomb core body in the metal casing and then uniting them together, namely, in the so-called canning cost, whereby a substantial cost merit can be brought about.
Neither the conventional exhaust gas cleaning devices constructed of a honeycomb core body alone nor the conventional exhaust gas cleaning devices constructed of a honeycomb core body and a metal casing can however withstand long-term application for the following reasons. In directions perpendicular to the longitudinal axis of the honeycomb core body (hereinafter called "radial directions of the honeycomb core body"), to say nothing of the axial direction of the honeycomb core body (namely, the direction in which exhaust gas flows in and passes through), deformation forces are produced by thermal expansion and thermal stress occurred in an atmosphere of high temperature due to the elevated temperature of exhaust gas itself and heat generated by a catalytic reaction of unburnt gas. These deformation forces then concentrate in the vicinity of the outer periphery of the honeycomb core body or around the areas of contacts between the outer peripheral wall of the honeycomb core body and the inner peripheral wall of the associated metal casing by way of the elements (i.e., the planar band and corrugated band) of the honeycomb core body. Namely, the large deformation forces caused by the aforementioned thermal expansion and thermal stress concentrate in the vicinity of the outer periphery of the honeycomb core body or around the areas of contacts between the outer peripheral wall of the honeycomb core body and the inner peripheral wall of the associated metal casing. The planar band and/or corrugated band, which make up the honeycomb core body, undergo substantial cracking and breakage at such locations, and dropping or separation takes place at the areas of contact between the respective bands and also at the areas of contact between the honeycomb core body and the metal casing.
With a view toward improving the joining strength at the areas of contact between a planar band and a corrugated band, it has been known to use a planar band which includes curved concave sections having the same radius of curvature as the curved convex sections (i.e., ridges) of an associated planar band, in other words, a wavy band having the same radius of curvature [Japanese Patent Application Laid-Open (Kokai) No. 30651/1989]. Although it is the primary object of this technique to improve the joining strength at the areas of contact between the planar band and the corrugated band, the production of thermal stress by the difference in thermal expansion between the planar band and the corrugated band can also be reduced as a result. However, this reduction is still insufficient.
In order to make an exhaust gas cleaning device of this sort, especially, a honeycomb core body successfully withstand thermal deformations as described above, corrections are needed to the belief that it is only necessary to establish firm joining or unification between a planar band and an associate corrugated band, which make up the honeycomb body, or between the outer peripheral wall of the honeycomb body and the inner peripheral wall of an associated metal casing.